The invention relates to a cartridge piston according to the preamble of claim 1.
Cartridge pistons consist as a rule of a soft plastic, for example LDPE, in order to achieve the necessary sealing action toward the cartridge wall. However, such plastics are only resistant to a limited degree against certain materials forming the cartridge content, and are furthermore not sufficiently diffusion-tight. For example, the Plastic LDPE can be attacked by polyester resins that cause it to swell.
In order to avoid this, a covering disk, for example of polyamide, since polyamide is resistant against the aforementioned materials and is also diffusion tight, is arranged on the upper side of the piston. These covering disks can be flat or can be shaped corresponding to the cross-sectional shape of the piston surface. In both cases, such covering disks rest sealingly on the cartridge wall.
The known pistons have an annular groove which has a V-shaped cross section in the area facing the cartridge wall, the outer edge of which annular groove is designed as a sealing lip. When the cartridge content is pressed out of the cartridge due to pressure on the piston, then the cartridge swells. At the same time, the pressure applied to the cartridge content causes the outer leg of the V-shaped groove, and thus the sealing lip, to be pressed against the cartridge wall.
The above effect, however, does not occur when the piston surface is covered by a covering disk which seals off the cartridge toward the cartridge wall. When the cartridge swells due to the pressure applied to the cartridge content, the seal between the cartridge wall on the one side and the sealing lips of the covering disk and the piston on the other side is influenced so that cartridge content exits rearwardly past the piston.
The purpose is to design the cartridge in such a manner that on the one hand the piston is protected against the influence of the cartridge content and on the other hand the sealing function of the piston is guaranteed even when the cartridge swells.
The purpose is attained with the characteristics of claim 1. Advantageous developments are disclosed in the subclaims.